Keywords: reliability, assessment, simulation, probability, safety, serviceability, timber, strength.

Current European standards for timber structures [1] are based on the semi-probabilistic approach to structural reliability assessment represented by partial factors design (PFD). Each random variable is represented by a characteristic value, which is modified by partial factors. This approach does not allow direct evaluation of the probability of failure. Developments in computer technology enables the use of higher levels of assessment (fully probabilistic methods). One of these methods is the simulation-based reliability assessment method (SBRA) documented in publications [2,3,4]. The subject of this paper is a demostration of the potential of SBRA assessment of the reliability of timber components and joints using an example of a timber beam.

Structural timber is a building material of natural origin with many positive, but also negative features. An increase of moisture and durability of the load combination effect in time leads to the reduction of the strength of timber. This dependence is nonlinear. This effect is known as the duration of load (DOL) effect and is one of the most important features influencing timber strength. Many models of the DOL effect have been made during the last 50 years. One of the first outstanding investigations was by Wood [5] at FPL, Madison. The almost linear relationship between stress level and logarithmic time is comonly known as "The Madison Curve".

In the SBRA method all input values are expressed by variables represented by bounded histograms, the loading is expressed by load duration curves, the reliability function (RF = R - S) is analysed using the direct Monte Carlo method and the reliability is expressed by comparing the calculated probability of failure P_f and the target probability P_d given in standards (e.g. CSN 73 1401/1998 [6]).

The safety assessment using the SBRA method is based on load effect combination analysis, the reduction of strength according Madison curves, the analysis of the safety function and the simulation technique.

A different approach can also be used for the verification of timber structures in ultimate limit states when using the SBRA method. This approach is based on load effect combination analysis, the Madison curves, the accumulation of damage procedure and the simulation technique.

The SBRA method introduces a new qualitative approach to structural design corresponding to the computer era. It provides the designer with a better understanding of the actual safety and serviceability of timber components and joints. The application of the Monte Carlo simulations technique corresponds to the potential of computers now available to all designers.

1

Eurocode 5, "Design of timber structures - Part 1.1: General - Common rules and rules for buildings", European Committee for Standardization, 2004.

2

P. Marek, M. Guštar, T. Anagnos, "Simulation-Based Reliability Assessment for Structural Engineers", CCR Press, Boca Raton, Florida, ISBN 0-8493-8286-6, 1995.

3

P. Marek et al., "Probabilistic Assessment of Structures using Monte Carlo Simulation. Background, Exercises and Software, (2nd edition)", ITAM CAS, Prague, Czech Republic, ISBN 80-86246-19-1, 2003.

4

A. Lokaj, "Timber structures assessment using SBRA method", Doctoral dissertation, Technical university of Ostrava, Czech Republic, 2001.

5

L.W. Wood, "Relation of stregth of wood to duration of load", U.S. Forest Products Laboratory, Report No. 1916, 12/1951, reprinted 1962.

6

CSN 73 1401, "Design of steel structures", (in Czech), CNI, Prague, 1998.

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